1. Field of the Invention
This invention is in the field of transparent heat mirrors for transmitting visible radiation and reflecting infrared radiation, and more particularly relates to transparent heat mirrors on polymeric substrates, such as flexible polymeric films.
2. Description of the Prior Art
Transparent heat mirrors are one class of spectrally selective coatings. They have optical properties which enable them to have high transmission of visible radiation and high reflectivity of infrared radiation. Such transparent heat mirrors have important applications as transparent thermal radiation shields with potential applications in solar energy collection, window insulation, etc.
One class of materials proposed for the formation of transparent heat mirrors is tin-doped indium oxide films. See, for example, Fan, J. C. C. and Bachner, F. J., Applied Optics, Vol. 15, No. 4, April 1976, pp 1012-1017; Fan, J. C. C., Bachner, F. J. and Foley, G. H., Applied Physics Letters, Vol. 31, No. 11, December 1977, pp 773-775; and Fan, J. C. C. and Bachner, F. J., J. Electrochem. Soc.: Solid-State Science and Technology, Vol. 122, No. 12, December 1975, pp 17-9-1725.
As evidenced by the afore-mentioned publications, tin-doped indium oxide films having sufficient optical properties to make them useful as transparent heat mirrors have been formed. Typically, however, such films have required either substrate temperatures during the deposition process exceeding the temperature resistance properties of most polymer film or a post-treatment, such as annealing, also requiring temperatures beyond those which most polymer films can tolerate. Almost all previous work in regard to these films involved radio frequency diode sputtering, which has too much intrinsic heating and electron bombardment for most available polymeric substrates. These obstacles to obtaining tin-doped indium oxide film of good wave-length selective properties on polymeric film have been recognized in the art. See, for example, Fan, J. C. C., Solid-State Chemistry of Energy Conversion and Storage, Advances in Chemistry Series 163, American Chemical Society, 1977, pp 149-164; and Sobajima et al., Proc. 6th International Vacuum Congress, 1974, Japan J. Appl. Phys. Suppl. 2, Pt. 1, 1974, pp 475-478.